On Tue, Dec 7, 2021 at 2:10 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > On Mon 06-12-21 10:35:03, Suren Baghdasaryan wrote: > > On Mon, Nov 29, 2021 at 3:23 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > > > On Wed 24-11-21 15:59:05, Suren Baghdasaryan wrote: > > > > oom-reaper and process_mrelease system call should protect against > > > > races with exit_mmap which can destroy page tables while they > > > > walk the VMA tree. oom-reaper protects from that race by setting > > > > MMF_OOM_VICTIM and by relying on exit_mmap to set MMF_OOM_SKIP > > > > before taking and releasing mmap_write_lock. process_mrelease has > > > > to elevate mm->mm_users to prevent such race. Both oom-reaper and > > > > process_mrelease hold mmap_read_lock when walking the VMA tree. > > > > The locking rules and mechanisms could be simpler if exit_mmap takes > > > > mmap_write_lock while executing destructive operations such as > > > > free_pgtables. > > > > Change exit_mmap to hold the mmap_write_lock when calling > > > > free_pgtables. Operations like unmap_vmas() and unlock_range() are not > > > > destructive and could run under mmap_read_lock but for simplicity we > > > > take one mmap_write_lock during almost the entire operation. Note > > > > also that because oom-reaper checks VM_LOCKED flag, unlock_range() > > > > should not be allowed to race with it. > > > > In most cases this lock should be uncontended. Previously, Kirill > > > > reported ~4% regression caused by a similar change [1]. We reran the > > > > same test and although the individual results are quite noisy, the > > > > percentiles show lower regression with 1.6% being the worst case [2]. > > > > The change allows oom-reaper and process_mrelease to execute safely > > > > under mmap_read_lock without worries that exit_mmap might destroy page > > > > tables from under them. > > > > > > > > [1] https://lore.kernel.org/all/20170725141723.ivukwhddk2voyhuc@xxxxxxxxxxxxxxxxxx/ > > > > [2] https://lore.kernel.org/all/CAJuCfpGC9-c9P40x7oy=jy5SphMcd0o0G_6U1-+JAziGKG6dGA@xxxxxxxxxxxxxx/ > > > > > > > > Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx> > > > > --- > > > > changes in v2 > > > > - Moved mmap_write_unlock to cover remove_vma loop as well, per Matthew Wilcox > > > > > > > > mm/mmap.c | 16 ++++++++-------- > > > > 1 file changed, 8 insertions(+), 8 deletions(-) > > > > > > > > diff --git a/mm/mmap.c b/mm/mmap.c > > > > index bfb0ea164a90..f4e09d390a07 100644 > > > > --- a/mm/mmap.c > > > > +++ b/mm/mmap.c > > > > @@ -3142,25 +3142,27 @@ void exit_mmap(struct mm_struct *mm) > > > > * to mmu_notifier_release(mm) ensures mmu notifier callbacks in > > > > * __oom_reap_task_mm() will not block. > > > > * > > > > - * This needs to be done before calling munlock_vma_pages_all(), > > > > + * This needs to be done before calling unlock_range(), > > > > * which clears VM_LOCKED, otherwise the oom reaper cannot > > > > * reliably test it. > > > > */ > > > > (void)__oom_reap_task_mm(mm); > > > > > > > > set_bit(MMF_OOM_SKIP, &mm->flags); > > > > > > Why do you keep this in place? > > > > Sorry for the delay, I was out last week. > > I missed your comment about removing MMF_OOM_SKIP at > > https://lore.kernel.org/all/YYrO%2FPwdsyaxJaNZ@xxxxxxxxxxxxxx > > I'll look into removing it in a separate patch, which I think would be cleaner. > > The point of this code was to sync up the oom_repaer and exit_mmap. Now > that your patch uses proper locking for that to happen then MMF_OOM_SKIP > is not really necessary. IIRC all you need to guarantee is that the vma > tree is empty when exit_mmap does all its work - i.e set mm->mmap to > NULL. You can do that after remove_vma loop but it would be equally safe > at any time after vma = mm->mmap as the loop relies on the vma chain. > Doing that after would be slightly nicer if you ask me. Will do. But if you don't mind I'll post the removal of MMF_OOM_SKIP as a separate patch. This patchset has already been extensively tested and it will be easier for me to test MMF_OOM_SKIP removal separately. > > > > Other than that looks OK to me. Maybe we want to add an explicit note > > > that vm_ops::close cannot take mmap_sem in any form. The changelog > > > should also mention that you have considered remove_vma and its previous > > > no MM locking assumption. You can argue that fput is async and close > > > callback shouldn't really need mmap_sem. > > > > Should I post another version of this patch with the patch description > > clarifying these points and additional comments as you suggested? > > Yes please. Will re-post. So, to clarify, we want: - Patch description to include explanation that remove_vma is now being called under MM lock but this should not be a problem because fput and vm_ops->close do not and should not take mmap_sem. - Add a comment for vm_ops->close that the callback should not take mmap_sem, with explanation that __split_vma and exit_mmap use this callback with the mmap_sem write lock taken. Is that correct? > > -- > Michal Hocko > SUSE Labs
